The aim of this paper is to underline the role of Sir James Mackenzie in stating that atrial fibrillation is a distinct and clinically important arrhythmia and that it is a common condition in patients with cardiac disease. Around 1900, a few clinical researchers were dealing with cardiac arrhythmias with the use of arterial and venous pulse tracings. Sir James Mackenzie, who has been one of the fathers of modern cardiology, introduced registration of the venous pulse at the bedside using the clinical polygraph he had invented. He applied the results of his experimental and clinical discoveries to the explanation of many kinds of arrhythmias before ECG introduction. In our paper, we have especially considered the three steps of his approach to atrial fibrillation, the first two developed in the pre-ECG era. The invention of an instrument of precision, the electrocardiograph, revolutionized the diagnosis of heart disease and catalyzed the formation of cardiology as a specialty.